Soil salinity causes significant reductions in plant productivity, and consequent economic losses associated with reduced grain quality and yield of agricultural crops (Pitman and Läuchli, 2002). Over 6% of the world's land is affected by either salinity or sodicity. A large proportion of the Australian wheat belt is at risk of salinisation due to rising water tables, and a further and larger part has soils that are sodic, and underlain with subsoil salinity (Rengasamy, 2002). This subsoil salinity is formed in semi-arid zones (with annual rainfall less than 450 mm), and is transient in nature as it moves in and out of the root zone according to soil wetting and drying cycles (Rengasamy, 2002).
Cultivars of durum wheat are more salt sensitive than bread wheat (Gorham et al. 1990; Rawson et al. 1988), and may yield less when grown on saline soils (Francois et al. 1986; Maas and Grieve, 1990). The usual high price of durum wheat on the international market can bring a better return to farmers than bread wheat and other crops, so, breeding new cultivars of durum wheat with improved salt tolerance can allow growers more options in dealing with subsoil salinity. Marker assisted selection is potentially the most efficient approach to developing cultivars that can tolerate saline soils.
There are three avenues by which to introduce salt-tolerance into durum wheat: traditional breeding techniques using physiologically-based phenotyping, marker-assisted selection, and through transformation of genes known to improve Na+ exclusion or tissue tolerance. To increase salt tolerance of crops in terms of yield increases and associated economic gains, there is great potential for the introduction of salt tolerance traits into durum wheat using marker-assisted selection (Munns et al. 2002). This approach has successfully been used to introduce various agronomic traits into cereals, and overcomes the problems associated with wheat transformation and market acceptance (Koorneef and Stam, 2001). Plant breeding using marker-assisted selection has a proven track-record of successfully incorporating a stable trait into the genome of the target species. However, marker development is dependent on accurate phenotyping, requiring a quantitative measure of a specific trait. An understanding of physiological mechanisms is needed to identify such a trait.
Salt tolerance in the Tritiaceae is associated with sodium exclusion, which limits the entry of sodium into the plant and its transport to leaves. Sodium exclusion from the transpiration stream reaching the leaves is controlled at three stages: (1) selectivity of the root cells taking up cations from the soil solution, (2) selectivity in the loading of cations into the xylem vessels in the roots, and (3) removal of sodium from the xylem in the upper part of the roots and the lower part of the shoot (Munns et al. 2002; Tester and Davenport 2003).
Bread wheat (hexaploid) cultivars are able to exclude Na+ from the leaves, however, durum wheat (tetraploid) cultivars lack this trait (Dubcovsky et al. 1996). The Kna1 locus on chromosome 4DL of hexaploid wheat is linked to Na+ exclusion and K+/Na+ discrimination, and subsequently, improved salt tolerance (Dvo{hacek over (r)}ák et al. 1994; Shah et al. 1987). Hexaploid wheat has three genomes, A, B and D, but tetraploid wheat has only the A and B genomes. A homoeologue of the Kna1 locus has not yet been found on the A or B genomes.
Recently, a novel source of Na+ exclusion was identified in a durum landrace (Munns et al. 2000). The landrace had very low rates of Na+ accumulation in the leaf blade, as low as bread wheat cultivars, and maintained a high rate of K+ accumulation, with consequent high K+/Na+ discrimination. The low-Na+ durum landrace had a K+/Na+ ratio of 17 whereas the durum cultivars Wollaroi, Tamaroi and Langdon had K+/Na+ ratios of 1.5, 0.7 and 0.4 respectively (Munns et al. 2000). The bread wheat cultivars Janz and Machete had K+/Na+ ratios of 10 and 8 respectively. The low Na+ trait was shown to confer a significant yield advantage at moderate soil salinity (Husain et al. 2003), indicating that this novel germplasm provides the opportunity to improve the salt tolerance of cultivated durum wheat. Markers for identifying the Nax1 locus from durum landrace wheat which is partially responsible for the sodium exclusion phenotype have recently been described (WO 2005/120214).
Methods for selection of Na+ excluding individuals in wheat breeding populations are time-consuming and expensive. In one case, the method involves growing plants in pots using a sub-irrigation system to provide a gradual and uniform exposure to NaCl to the plant, and the harvesting of a given leaf for Na+ accumulation. Although this screening method is very reproducible, it is labour intensive and requires a controlled environment. It is not possible to screen plants in the field or with large numbers of individual lines using this method. QTL mapping and marker-assisted selection is a technique that has many advantages over phenotypic screening as a selection tool. Marker-assisted selection is non-destructive and can provide information on the genotype of a single plant without exposing the plant to the stress. The technology is capable of handling large numbers of samples. Although developing a QTL map is laborious, the markers identified may prove to be sufficiently robust to use as the sole selection tool for a specific trait in a breeding program. PCR-based molecular markers have the potential to reduce the time, effort and expense often associated with physiological screening. In order to use marker-assisted selection in breeding programs, the markers must be closely linked to the trait, and work across different genetic backgrounds.
There is a need for the identification of further genes and/or markers thereof which can be used to produce plants, particularly wheat or barley plants, with enhanced tolerance to saline and/or sodic soils, and/or reduced sodium accumulation in an aerial part of the plant.